Hide the "hidden" fields on custom form previews

Summary:
Hide the "hidden" fields on custom form previews.

Before this change, Phabricator's custom form preview doesn't actually hide the
hidden fields, instead it shows them at 50% opacity. It looks cluttered and
doesn't provide a very useful "preview" at all.

This just hides some fields via CSS.

Cherry picked from:

https://phabricator.wikimedia.org/rPHABbba62cf5243538af9e37cc1211a01d247294f9f7

Upstream Task:

https://phabricator.wikimedia.org/T209743

Test Plan: Tested in Wikimedia's fork. I believe it results in better UX.

Reviewers: #blessed_committers, O1 Blessed Committers, Cigaryno, avivey, Matthew, valerio.bozzolan

Reviewed By: #blessed_committers, O1 Blessed Committers, Cigaryno, avivey, valerio.bozzolan

Subscribers: avivey, Cigaryno, speck, tobiaswiese, valerio.bozzolan, Matthew

Maniphest Tasks: T15081

Differential Revision: https://we.phorge.it/D25037